A research group in China has developed a new recycling process for end-of-life crystalline silicon (c-Si) PV modules based on three main stages: heavy-liquid separation of mixed materials, solar-cell etching, and solder-strip etching. The researchers also conducted a life cycle assessment (LCA) and techno-economic analysis (TEA) to evaluate the process.
“Through systematic experimental investigations, the core reaction mechanisms involving redox reactions, complexation equilibrium, and hydrolysis precipitation were elucidated, providing a theoretical foundation for the development of similar recycling processes,” the researchers said. “The selection of green chemical reagents, superior recovery performance, and closed-loop recycling potential of reagents reduce the environmental impact of the process and lay a solid foundation for its industrial application.”
The team used a mixture of glass particles, solar cells, and solder strips supplied by a recycling company. In the first stage, the materials were separated using a zinc bromide (ZnBr₂) heavy liquid. By adjusting the liquid density, the researchers induced different fractions to either float or sink, enabling separation of the material streams. The process recovered more than 98% of the solar cells and almost all solder strips prior to further treatment.
In the second stage, the separated solar cells were treated with a solution of aluminum chloride hexahydrate (AlCl₃·6H₂O) and hydrogen peroxide (H₂O₂) under hydrothermal conditions. The process removed the silver contacts, aluminum back layer, and silicon nitride (Si₃N₄) anti-reflective coating while preserving the underlying silicon wafer.
After optimizing process parameters, the researchers identified the best operating conditions as an AlCl₃·6H₂O concentration of 1.2 mol/L, an H₂O₂ concentration of 2.0%, a reaction temperature of 200 C, and a treatment time of 120 minutes.
In the third stage, the separated solder strips were treated with a copper chloride dihydrate (CuCl₂·2H₂O) solution. The strips consisted of a copper core coated with a lead-tin (Pb-Sn) alloy. The aim of this step was to remove lead and tin while preserving the copper core.
The team optimized CuCl₂ concentration, stirring speed, reaction time, and temperature, identifying 0.4 mol/L CuCl₂·2H₂O, 600 rpm, 15 minutes, and 60 C as the optimal conditions.
The process produced silicon with a purity of 99.997%, silver chloride (AgCl) with a purity of 99.64% and a silver recovery efficiency of 80.07%, recovered aluminum in solution, and copper strips with a purity of 99.99%. It also generated tin oxide (SnO₂) and lead sulfate (PbSO₄) from solder-strip byproducts. In addition, the CuCl₂ etching solution was successfully regenerated and reused, further improving the process’s sustainability.
The researchers then performed an LCA using a functional unit of 1 kg of waste input for each of the three stages. The heavy-liquid separation, solar-cell etching, and solder-strip etching steps showed global warming potential (GWP) contributions of 0.049 kg CO₂-eq, 3.522 kg CO₂-eq, and 0.055 kg CO₂-eq, respectively. Compared with conventional treatment methods, the process reduced carbon emissions by 80.42%, according to the analysis.
“Economic feasibility results show that the recycling profits of the heavy-liquid separation, solar-cell treatment, and solder-strip treatment steps are -$0.04/kg, $7.76/kg, and $4.81/kg, respectively,” the researchers said.
They attributed the negative profit in the heavy-liquid separation stage to the accounting methodology used in the analysis.
“Only the recovery value of glass was attributed to this step in the calculation, while the economic values of the separated solar cells and solder strips were assigned to their corresponding treatment steps,” they explained.
The novel technique was presented in “Sustainable recycling of waste crystalline silicon photovoltaic modules based on heavy liquid separation and metal chloride etching,” published in the Journal of Cleaner Production. Researchers from Sun Yat-sen University in China and the China University of Mining and Technology have contributed to the study.
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